Anaerobic treatment of industrial wastewater

Abstract

Feed water is processed in an anaerobic digester. A solid-liquid separation device, for example a sludge screw thickener, treats a stream drawn from the digester in a recirculation loop. The solids portion is returned to the digester to increase the solids retention time and the TSS concentration in the digester. A liquid portion with less than 5% of the solids in the stream is removed and optionally treated further. The flow rate to the solid-liquid separation device is preferably greater than the influent flow rate. The solid-liquid separation device may receive digestate at a TSS concentration of 4% or more and return a solids portion having a TSS concentration of over 10%. The feed water is preferably one or more industrial waste streams having a COD concentration of 20,000 to 50,000 mg/L and a TSS concentration from 1-5%. The organic loading rate may be 10-12 kg/COD/m3/day.

1 . A method for treating wastewater comprising steps of,
 a) feeding an influent at a flow rate Q to an anaerobic digester;   b) feeding a mixture from the anaerobic digester a flow rate of Q or more to a solid liquid separation device;   c) separating the mixture into a liquid portion and a solids portion; and,   d) returning at least most of the solids portion to the anaerobic digester.   
     
     
         2 . The method of  claim 1  wherein the solid liquid separation device is a screw thickener. 
     
     
         3 . The method of  claim 1  wherein substantially all of the solids portion is returned to the anaerobic digester. 
     
     
         4 . The method of  claim 1  wherein the anaerobic digester has a TSS concentration of 4 to 7%. 
     
     
         5 . The method of  claim 1  wherein the solids portion has a TSS concentration of over 10%. 
     
     
         6 . The method of  claim 1  wherein the solids portion has a TSS concentration of over 12%. 
     
     
         7 . The method of  claim 1  wherein the solids portion has a TSS concentration in the range of 12-14%. 
     
     
         8 . The method of  claim 1  wherein the organic loading rate of the anaerobic digester is at least 10 kg/COD/m3/day. 
     
     
         9 . The method of  claim 1  wherein the influent has a chemical oxygen demand concentration of about 20,000 to 50,000 mg/L and a total suspended solids concentration of about 1-5%. 
     
     
         10 . The method of  claim 1  wherein the solids portion contains 95% or more of the suspended solids in the mixture. 
     
     
         11 . The method of  claim 1  wherein the solids portion has a TSS concentration that is at least twice the TSS concentration of the mixture. 
     
     
         12 . The method of  claim 1  further comprising a step of treating the liquid portion in a second solid liquid separation device. 
     
     
         13 . The method of  claim 12  wherein the second solid liquid separation device is selected from the group consisting of a dissolved air flotation unit, a cavitation air flotation unit, and a tubular membrane. 
     
     
         14 . The method of  claim 12  wherein the second solid liquid separation device is a dissolved air flotation unit. 
     
     
         15 . The method of  claim 1  further comprising a step of treating the liquid portion in an activated sludge reactor. 
     
     
         16 . The method of  claim 15  wherein the activated sludge reactor is a membrane bioreactor. 
     
     
         17 . The method of  claim 1  further comprising a step of treating the liquid portion in an anaerobic membrane bioreactor. 
     
     
         18 . The method of  claim 1  further comprising a step of adding a polymer to the mixture. 
     
     
         19 . A system for treating an influent comprising,
 a) an anaerobic digester; and,   b) a re-circulation loop from the anaerobic digester to a screw press and back to the anerobic digester.   
     
     
         20 . The system of  claim 19  wherein the screw press has a screening section that may be selectively fixed or allowed to rotate. 
     
     
         21 . The system of  claim 19  wherein the screw press has a screening section with openings in the range of 200 to 500 microns or made from wedgewire with a slot opening in the range from about 0.25 mm to about 0.75 mm. 
     
     
         22 . The system of  claim 19  wherein the screw press has an auger shaft with an increased diameter towards the outlet end of the screw press. 
     
     
         23 . The system of  claim 19  wherein the screw press has a sprayer system adapted to spray water against the outside of a screening section. 
     
     
         24 . The system of  claim 19  wherein the screw press has an enclosed screening section.